Manufacture process of permanent magnets from sintered mixtures of oxides



March 25,-- 1958 s. MEDVEDIEFF 2,328,254

MANUFACTURE PROCESS OF PERMANENT MAGNETS FROM SINTERED MIXTURES OFOXIDES Filed Sept. 12, 1955 I Fl-G.2

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SCRGE an/501s FF NYVE/V TOE MM A r-roomy United States PatentMANUFACTURE PROCESS OF PERMANENT MAGIEISTITS FROM SINTERED MIXTURES OF0X11) Serge Medvedieif, Boulogne sur Seine, France, assignor to SocietyAudax, Montreuil sous Bois, France, a French society ApplicationSeptember 12, 1955, Serial No. 533,577

Claims priority, application France November 9, 1954 Claims. (Cl.252-625) The invention relates to permanent magnets constituted bysintered mixed oxides of iron and of one or more of the metals barium,strontium, calcium and lead, incorporating oxide of chromium.

The ferromagnetism of the ferrites of the alkaline-earth metals and oflead has been known for several decades.

It has been shown that these ferrites are very ferromagnetic and thatthey have an hexagonal structure.

The employment of the ferrites of Ba, Sr, Ca and Pb of the formula nFe OMO, in which n is a number between 2 and 9 and M denotes one of theaforesaid metals, for the manufacture of permanent magnets has beenproposed in the specification of French Patent No. 1,048,792 of July 18,1951. In that specification, it appears that the appearance of theproperties of a permanent magnet in these ferrites is attributed to the.fact that the majority of the grains of the sintered aggregate would beof small dimensions as they comprise only a single Weiss domain; sincethese ferrites are characterized by a considerable magnetocrystallineanisotropy, the dimensions of the single domain particles might be ofthe order of microns.

This interpretation appears to be erroneous, because the powder ofbarium ferrite, for example, in the sintered state renders it possibleto obtain compact magnets having a density-of approximately 5, thetheoretical density corresponding to zero porosity'being 5.3 gms. percc. In this advanced state of aggregation, it may be supposed that therecould not be grains which are magnetically insulated from one another.

It is more probable that the magnetic properties of the ferrites depend,to-a certain extent, on the residual porosity of the sintered pieces. Ithas already been observed that, in the case of the magnets made ofcobalt ferrite, a too advanced sintering involved a rapid diminution ofthe coercive force and of the. residual magnetisation (S. Medvedieff,These 'dIngenieur-Docteur, Paris, 1952).

In order to obtain a high residual magnetisation B,, it is necessary togive the sintered material a density that is as great as possible. Infact, if 0 denotes the magnetic moment per unit of mass, themagnetisation I per unit of volume is equal to the product 1rd, where 0!denotes the density, and the residual induction B is equal to that is tosay, isproportional to d.

Now, experience shows that,'for large values of the density (of theorder of about 5), an increase thereof if accompanied by a rapiddiminution of the coercive force 1 The heat treatments then becomeextremely critical. By way of example, on heating the same powder of 6FeO BaO for one hour at temperatures of 1150" C. and 1200 C., residualinductions of 2350 and 2300 gauss respectively and coercivemagnetisation forces (values of the magnetic field eliminating themagnetisation) dispersed between 1800 and 1050 oersteds have been found.

N 2,828,264 Patented Mar. 25, 1958 From applicants experiments it hasbeen found that the mixed oxides of iron, of chromium and of analkalineearth metal or lead, render it possible, owing to the presenceof chromium oxide, to produce permanent magnets,

having, at the same time, highvalues of residual magnetisation and ofthe coercive force. These magnets have,

for a density of approximately 5, verygreat coercive in the mass of thematerial and favouring the existence of a considerable coercive forceowingto 'a mechanism such as that known in the theory of Kerstensforeign bodies (M. Kersten,,Hirzel, Leipzig, 1943, pp. 1-88).

On the contrary, the chromium ions form an integral part of thecrystalline lattice of the mixed oxides formed at temperatures ofbetween 900 C. and 1400 C. as is confirmed by the displacement of theCurie point.v Oxides of .iron (Fe O and of chromium (Cr O and bariumcarbonate (BaCO have been mixed in proportions that are suitable forobtaining the molecular ratios Fe/Cr hereinafter indicated, and thenfinely ground in a ball mill for fourteen hours. The mixture was dried,sifted and compressed into the form of rods, having a diameter of 1 cm.and a length of from 1 to 2 cms., which were baked at 1200 C. for onehour. With the proportion of barium oxide remaining the same, the Curiepoint was taken for different proportions Fe/Cr:

Curie point, degrees C.

In the case in which the alkaline-earth metal is barium, the magnetsaccording to the invention can be prepared by using, as initialmaterial, ferric oxide (Fe O and barium chromate. An intimate mixture ofthese compounds in the suitable proportions .is made by grinding Thepowder is dried,

for fourteen hours in a ball mill. sifted, a binder being added, ifrequired, which is entirely eliminated on heating, and then compressedby an hydraulic press under a pressure of 2 to 4 tons per squarecentimeter into the form of rods having a diameter of 1 cm. and a lengthof from 1 to 2cms. These rods are baked for one hour at 1200 C;

By way of example; the following four mixtures have been prepared:

Example 1.-.Fe O 192 grams; BaCrO 51 grams,

Fe/Cr=12. Example 2.Fe' O 240 grams; BaCrO 76 grams,

Fe/Cr=10.

Example 3.Fe O 192 grams; BaCrO 76 grams,

Fe/C r= 8. p v Example 4.--Fe O 144 grams; BaCrO 76' grams,

In .order to show that the conditions of sintering are not critical, oneportion of the rods obtained with the powders of the different exampleswere baked for one" I hour. at 1150 Cflandfanother portion were bakedfor 3 l the same period at 1250* C. The features of the magnets obtainedare set forth in the following table:

The range of molecular proportions Fe/Cr hereinbefore given as between 6and 12 may be greater, for example between 4 and 20.

From the magnetic point of view,'the crystals of the mixed oxides ofiron, of chromium and of an alkalineearth metal are magneticallyanisotropic and have (1) A magnetic anisotropy due to the existence ofan axis of easy magnetisation which, in the present case, is the senaryaxis.

- (2) An anisotropy of shape, which, in the case of needle-shapedcrystals, is proportional to the difference between the demagnetisingshape factors along the geometrical axis of the needle and in aperpendicular direction.

Also, grains of the compound according to the invention, which areplaced in an homogeneous magnetic field, become aligned in a directionwhich is the resultant of the two above anisotropy axes; if thecrystallisation of the initial mixture has been suitably regulated, theproduct obtained is composed of essentially mono-crystalline fineparticles for which the axes are coincident.

As has been shown by numerous studies and, in particular, by those ofCharles Guillaud, Proprits daimant des poudres ferromagntiques, Journaldes Recherches du Centre National de la Recherche Scientifique, Nov. 9,1949, it appears, and experience confirms, that the properties ofmagnets made of magnetic material in highly anisotropic fine grains aregreatly improved if the grains are aligned.

C. Guillaud has demonstrated it on a powder of the alloy Mn-Bi. Thisalloy has the same hexagonal structure as the compound of the inventionand is characterised by a high magnetic anisotropy along the senaryaxis. He has, by making the following experiments, shown that thisalloy, reduced to the state of a fine powder, lent itself to theproduction of excellent permanent magnets by compressing the powder.

(1) The powder was compressed without regard to parallel alignment ofthe magnetic axes of the particles of the alloy.

(2) The powder was compressed (or, more accurate- 1y, set by cooling amixture of powder and wax) in the presence of an homogeneous magneticfield.

In the second case, the residual magnetism is three times as great as inthe first case.

According to an improvement of the invention, permanent magnets are madewith the powders hereinbefore described by aligning the grains of thesepowders before the sintering treatment.

The manufacture of the permanent magnets according to the invention iscarried out as follows:

Ferric oxide and barium chromate are mixed in one of the foregoingproportions, for example in the proportion Fe O 1BaCrO The mixture isground in water in .a ball mill for to 14 hours. The ground mixture isdrained and dried and then compresed into the form of cakes having, forexample, the following dimensions: diameter 8 centimeters, height 2 to 3centimeters.

These cakes are baked at 1250 C. for one hour in air and then soaked inwater. The lump of oxides bursts and the fragments are friable and canbe crushed easily to a fineness for enabling it to pass through a No. 25sieve.

This coarse powder is ground with water for a time of between 4 and 14hours, for example, for six hours.

After grinding, the paste or sludge is kept and, after addition of asuitable binder, is used for preparing oriented magnets.

Two methods of orientation may be employed.

In order to obtain'a radial magnetisation, that is to say magnets 14having the shape of a cylindrical ring (torus having a rectangularcross-section) oriented with a given polarity on the internal cylinder15 and with opposite polarity on the external cylinder 16 (Fig. 1), theapparatus represented in Fig. 2 is employed.

The apparatus comprisesa coil 1, through which a direct current passes,and a magnetic circuit comprising a central hollow post 2 constitutingthe core of the coil, a cylindrical part 3 coaxial with the said post, abase 4 connecting the post 2 with the cylinder 3, and a lid 5 havingtherein a hole in which a ring 6 is fitted. The central post 2 issurmounted by a pole head 7 which, with the ring 6, bounds an air gap inwhich the lines of force are radial.

, Beneath the lid 5 there is arranged a plate 8 made of bronze or othernon-magnetic metal and having holes 9 therein. Arranged above the coilis a second plate 10 also made of non-magnetic metal and, together withthe former plate, defining an exhaust chamber 11. The plate 10 isfunnel-shaped and is joined to the central post 2 in a zone in whichholes 12 bring the exhaust chamber 11 into communication with thepassage 13 provided in the central post. The passage 13 may be put intocommunication with a vacuum pump.

The bottom of the annular space between the members 6 and 7 is coveredwith a filter cloth and the mould thus formed is filled with paste. Theradial magnetic field is applied by passing current through the coil '1and the water is removed by putting into operation the vacuum pumpconnected with the passage 13.

Experience has shown that, in order to ensure the appropriate alignmentof the grains of the powder along the lines of force of the magneticfield, it was necessary that the grains should be in suspension in aliquid, for example water, so that they should be sufficiently mobile.The grains are thus oriented in the liquid phase. As the water isremoved, the position of the grains remains fixed by the field. When thegreater part of the water hasbeen removed, there is applied, to the toppart, with the aid of a piston 28 made of non-magnetic metal, a pressureof some hundreds of kgs. per square centimeter which compresses thethick paste.

The field is cut 0E and the assemblage comprising the members 6 and 7and the paste is removed; After a partial drying of the compressedpaste, the latter is subjected, in the mould itself, to a compression ofthe order of one ton per square centimeter. -After removal from themould, a piece in the form of a ringis obtained, one of the poles ofwhich is spread over the inner cylindrical surface and the other isspread over the outer cylindrical surface. The particles are thereforeoriented radially. The compressed piece is then subjected to baking inan electric furnace for a time varying from half an hour to one hour andto a slow cooling. A sintered ceramic piece is thus obtained which has adensity of between 4.5 and 5 gms. per cubic centimeter and which, if'theradial field applied is between 4000 and 5000 oersteds, gives a powerfulmagnet with radial magnetisation, the characteristics of which aresubstantially Residual induction: 13,:2900-3300 gauss. V Coercive forceof'magnetisation I ==16002500 oerteds. p

(BH) max.=1.7 to 2.2.10 gauss-oersteds.

In order to obtain a parallel magnetisation, that is to say magnets inthe form of parallelepipeds or of discs 17 (Fig. 3), the north and southpoles of which are situated respectively on two parallel faces 18 and19, the apparatus represented in Fig. 4 is employed.

The apparatus comprises a magnetic circuit 20 which produces a magneticfield of some thousands of oersteds between the plane faces opposite anupper piston 22 and a bottom cylindrical stand 23 made of steel. Thepiston 22 slides inside a thick tube 24 which is made of nonmagneticmetal and in which the stand 23 is encased. The bottom stand 23 isprovided with holes 25 and is connected to a vacuum pump through piping26. A filter cloth 27 is arranged on the top face of this perforatedstand; the fluid paste is poured into the mould constituted by the tube24; the piston 22 is engaged in the tube, the field is applied and thevacuum pump is set in operation. The forces of magnetic attraction inthe ari gap exert a first compression on the paste and accelerate thefiltration. As the length of the air gap diminishes, the fields, towhich the particles are subjected, increases with the compression. Atthe end of a few minutes, the force of an hydraulic press, giving apressure of one ton per square centimeter, is applied to the piston.After removal from the mould, the disc of oriented material, althoughpossessing a cohesion which is sufficient for the manipulations (owingto the previously incorporated binder), is still damp. It is dried inorder to eliminate first the moisture and then the binder. Finally, itis subjected to baking under the same conditions as in the case of theradial magnetisation. The magnetic characteristics are the same.

The temperature and pressure values which have been given depend uponthe molecular ratio which, as has been seen, may vary from 4 to 20 andare not critical. A pressure of between 1 and 4 tons per squarecentimeter and a temperature of between 900 and 1400 C. are allowable.

What I claim is:

1. A process for preparing an anisotropic permanently magnetic bariumchromium ferrite having a density of at least aboutv 4.45 comprisingmixing Fe O with a member of the group consisting of BaCr and a mixtureof BaCO and Cr O as will form BaCrO the molecular proportion of iron tochromium being between 4 and 20, pressing said mixture to a temperatureof between about 900" C. and about 1400 C. and subjecting said pressedmixture to a steady magnetic field to provide a shaped permanent magnethaving a residual induction of at least about 1800 gauss and a coerciveforce of mag netization of at least about 3200 oersteds.

2. A process for preparing an anisotropic permanently magnetic bariumchromium ferrite having a density of at least about 4.45 and comprisingmixing finely divided Fe O and finely divided BaCrO the molecularproportion of iron to chromium being between 4 and 20 and pressing saidmixture at a temperature of between about 900 C. and about 1400 C. tosinter said mixture and to decompose said barium chromate into bariumoxide and chromium oxide and subjecting said sintered mixture to asteady magnetic field to provide a shaped permanent magnet having aresidual induction of at least about 1800 gauss and a coercive force ofmagnetization of at least about 3200 oersteds.

3. A process for manufacturing a permanently magnetic barium chromiumferrite made of mixed oxides as claimed in claim 1, comprising mixingfinely divided Fe O and fine divided BaCrO in proportions to obtain amix-ture in which the molecular proportion between iron and chromiumlies between 4 and 20, sintering said mixture while said mixture issubjected to a steady magnetic field, said mixture being sintered bypressing and heating at a temperature of between 1150 and 1400 C. underthe said field.

4. The product produced by the process of claim 1.

5. The product produced by the process of claim 2.

References Cited in the file of this patent UNITED STATES PATENTS1,997,193 Kato et al. Apr. 9, 1935 2,579,267 Leverenz et al. Dec. 18,1951 2,762,777 Went et al Sept. 11, 1956 2,762,778 Gorter et al Sept.11, 1956 OTHER REFERENCES Journal of Chemical Physics, vol. 15, No. 4,April 1947, pages 181-187, page 184 pertinent,

1. A PROCESS FOR PREPARING AN ANISOTROPIC PERMANENTLY MAGNETIC BARIUMCHROMIUM FERRITE HAVING A DENSITY OF AT LEAST ABOUT 4.45 COMPRISINGMIXING FE2O3 WITH A MEMBER OF THE GROUP CONSISTING OF BACRO4, THEMOLECULAR PROPORATION OF IRON TO CHROMIUM BEING BETWEEN LECULARPROPOTION OF IRON TO CHROMIUM BEING BETWEEN 4 AND 20, PRESSING SAIDMIXTURE TO A TEMPERATURE OF BETWEEN ABOUT 900*C. AND ABOUT 1400*C. ANDSUBJECTING SAID PASSING MIXTURE TO A STEADY MAGNETIC FIELD TO PROVIDE ASHAPED PERMANENT MAGNET HAVING A RESIDUAL INDUCTION NETIZATION OF ATLEAST ABOUT 3200 OERSTEDS.